Bioavailability of Arsenic and Lead in Soils from the Butte, Montana, Mining District Andy Davis” PTI Environmental Services, 2995 Baseline Road, Suite 202, Boulder, Colorado 80303
Mlchael V. Ruby PTI Environmental Services, 2995 Baseline Road, Suite 202,Boulder, Colorado 80303
Paul D. Bergstrom Atlantic Richfield Company,
555 17th Street, Denver, Colorado 80202
Risk assessments for mining sites are typically based on As and Pb bioavailability determined from studies using soluble metal salts. This is an inappropriate analogue of soil ingestion at mining sites due to the lower solubility of As and Pb in naturally occurring mineral assemblages. As and Pb bioavailability in soils from Butte, MT, is further reduced by authigenesis, where galena oxidizes to form a coating of anglesite further armored by sparingly soluble K-jarosite precipitates. Enargite and tennantite were also encapsulated by K-jarosite. When the soil was fed to New Zealand White rabbits, only 11%CASand 6% C P b were solubilized in the small intestine. In vitro tests demonstrate that Na2HAs0,.7H,0 and Pb(OAc)2overestimate As and P b solubility by at least a factor of 5. MINTEQAP accurately simulated experimental equilibrium As and P b concentrations, but overestimated their solubility for residence times typical of the gastrointestinal tract. Introduction
The toxicological effects of arsenic and lead have been studied extensively (1-6). However, the reason for the variable nature of observed blood P b levels in different communities with similar soil P b concentrations, i.e., at urban (9, smelter (B), and mine waste impacted sites (Figure l),has not been addressed. The toxicity of ingested Pb is dependent on its bioavailability, expressed as the percentage of ingested materials absorbed through the intestinal wall into the systemic blood system. Typically, 100% of ingested As and 30% or more of ingested Pb have been assumed to be bioavailable (6,9, IO),based on toxicological studies using pure As and P b salts. However, this approach is flawed if the objective is to evaluate the bioavailability of metals from soils, because solid phases in soil are typically much less soluble than metal salts. This discrepancy leads to an overestimate of metal bioavailability when soluble salts are utilized as an analogue for soil As and Pb-bearing compounds. In this paper, the term bioavailability is used to describe that portion of the ingested As and P b that is absorbed into the blood stream, while solubility will be used to 0013-936X/92/0926-0461$03.00/0
denote the equilibrium concentration of As or P b in solution in the small intestine. The term availability is used to describe time-dependent As and Pb concentrations (not necessarily equilibrium) that may be less than the solubility (i-e.,solubility > availability > ‘bioavailability). The availability of As or P b is a precursor to bioavailability because absorption of both metals occurs primarily from the fluid phase in the small intestine (11,12). Soil mineralogy is a critical parameter controlling mineral dissolution because oxidation reactions on mineral surfaces result in armoring of the primary mineral grain by a secondary reaction product. In addition, the As- or Pb-bearing phase is often encapsulated in an insoluble matrix (e.g., silica), further diminishing metal availability. Previous studies of Butte mineralogy have emphasized the bulk mineral percentages in the copper porphyry ore body (13) but authigenesis of mine waste related soil at a microscopic level has not been investigated. This paper describes the mineralogy of the As- and Pb-bearing solids in mine waste impacted soils, reports on metal solubility resulting from dissolution of these solids in the gastrointestinal (GI) tract (based on in vivo and in vitro experiments), and presents data that compare the representativeness of soluble salts and natural soils as estimators of metal availability. Finally, we used a geochemical model and experimentally determined dissolution rates to investigate whether dissolution of the solubilitycontrolling mineral phases was controlled by equilibrium or kinetic constraints. The research was conducted using soils from Butte, MT, a mining site from 1864 to the present. Methods
Sample Treatment and Mineralogical Analysis. Soil I, representative of a mine waste site minimally impacted by smelter activity, was blended from five separate Butte soils to achieve a desired As and P b content (1380 mg/kg As; 3900 mg/kg Pb). After collection, each of the five soils was air-dried and sieved to -
29 Pb2+ 6 PbOH' 63 Pb citrate 2 P b acetate 30 Pb2+ 5 PbOh+ 4 PbS04(aq) 59 P b citrate 2 Pb acetate anglesite 69 (10-3.77) 37 (10-339) 61 (10-49 45 (10-3.96) ge (10-4.74)
Acetate and citrate present at M; C1- present at M. Due to precipitation of PbS04. Numbers in parentheses, apbz+. Due to precipitation of Pb,On(SOa)and PbSO,. e Due to lead phosphate precipitates discussed in text.
Simulation of anglesite dissolution yields 69 mg/L P b in solution at pH 2.0, E h +200 mV, and 0.01 M C1-, decreasing to 37 mg/L as the digestate passes through the small intestine (pH 7.0). The addition of organic acids capable of complexing Pb2+(acetate and citrate at lo4 M) increases dissolved Pb to 61 mg/L in the small intestine, while decreasing the activity of Pb2+ion (Table V). The addition of Ca2+and Po4,- at M, in the absence of organics. decreases dissolved P b to 7.5 me/L. due to the p6cipitation of Pb5(P04),C1,Pb5(P04),(O%),'and Pb,(P-
0"L Galena dissolution at pH 2.0, E h +200 mV, and 0.01 M C1- was predicted to result in a dissolved P b concentration of 69 mg/L, controlled by anglesite precipitation. This result is consistent with the observation of galena alteration to anglesite (Figures 4, 8, and 9). At pH 7.0, dissolution of galena resulted in 10 mg/L P b in solution, due to precipitation of Pb403(S04)and PbS04. Although the presence of the PbC1,2-n complexes in the GI tract (27,37) has not been substantiated experimentally, the principal inorganic P b complexes resulting from equilibrium anglesite dissolution in the small intestine are predicted to be PbC1+ (63%), Pb2+(26%),PbClzo(5%), PbOH+ (3%), and PbSOd0(2%) (Table V). In the presence of citrate, a majority of P b (59%) will be complexed as lead citrate. It is likely that PbC1+ predominates in the stomach, and that the presence of organic ligands in conjunction with C1- activity will control the speciation of P b between Pb2+,lead chlorides, lead hydroxide, and lead organic species in the small intestine.
Discussion This study has demonstrated that As and Pb mineralogy from the reduced portion of the ore zone typically consists of primary sulfides (e.g., enargite and galena), altering to a restricted suite of secondary minerals (e.g., anglesite and plumbojarosite). At smelter sites, soil mineralogy is more diverse, consisting of remnant sulfides and multimetal oxide phases. Finally, roadside soils may contain high Pb concentrations in a readily soluble amorphous form from automobile emissions (Table I). These conclusions are consistent with the data of Foster and Lott (38),who studied the composition of lead compounds in airborne particulates associated with ore han-
dling, sintering, and blast furnace operations around a lead smelter in Missouri. Galena was reported as the major constituent in samples associated with ore handling, while lead oxide species (PbO,, Pb0.PbS04) were the dominant species associated with smelter operations. The geochemical factors controlling availability of metals from the soils include the mineral composition, the degree of encapsulation, the nature of the alteration rinds, and the rate of dissolution in the GI tract (directly related to particle size). In contrast to soluble metal salts, the dissolution of As- and Pb-bearing solids is limited by the refractory nature of arsenic sulfides and lead sulfates, indicating that site-specific mineralogy is necessary to characterize the availability of As and Pb. In addition, galena crystals are often enclosed in a pyrite or silicate matrix, making them unavailable for dissolution, while Pb phases frequently alter to anglesite, which encapsulates the primary minerals, shielding them from GI fluids. Precipitation of jarosite also retards As and Pb dissolution, both physically, by reduction in the exposed surface area, and chemically, due to the insoluble nature of jarosites in acidic (pH
